Imaging device

ABSTRACT

An imaging device for projecting individually controllable laser beams onto an imaging surface movable in an X-direction. The device includes a plurality of semiconductor chips each comprising a plurality of laser beam emitting elements arranged in a main array of M·N. The chips are mounted such that each pair of adjacent chips in the Y-direction are offset from one another in the X-direction and, if activated continuously, the emitted laser beams of the two chips of said pair trace on the imaging surface a set of parallel lines that are substantially uniformly spaced in the Y-direction. In addition to the M·N elements of the main array, each chip comprises at least one additional column on one or each side, each additional column containing at least one selectively operable element capable of compensating for any misalignment in the Y-direction in the relative positioning of the adjacent chips on the support.

RELATED APPLICATIONS

This patent application incorporates by reference in their entiretyInternational Patent Applications Nos. PCT/IB2016/053138 andPCT/IB2016/053137, filed on May 27, 2016, and GB Patent ApplicationsNos. 1509073.1 and 1509077.2, filed on May 27, 2015.

FIELD

The present disclosure relates to an imaging device for projecting aplurality of individually controllable laser beams onto a surface thatis movable relative to the imaging device.

BACKGROUND

U.S. Pat. No. 7,002,613 describes a digital printing system to which theimaging device of the present disclosure is applicable, by way ofexample. In particular, in FIG. 8 of the latter patent specification,there is shown an imaging device designated 84 that is believed torepresent the closest prior art to the present disclosure. The imagingdevice serves to project a plurality of individually controllable laserbeams onto a surface, herein termed an imaging surface, to generate anenergy image onto that surface. The laser image can be used for avariety of purposes, just a few examples being to produce a twodimensional printed image on a substrate, as taught for instance in U.S.Pat. No. 7,002,613, in 3D printing and in etching of an image onto anysurface.

For high throughput applications, such as commercial printing or 3Dlithography, the number of pixels to be imaged every second is veryhigh, demanding parallelism in the imaging device. The laser imagingdevice of the present disclosure is intended for applications thatrequire energy beams of high power where the total power required can beof tens or hundreds of milliwatt (mW). For instance, in the field ofprinting, depending on the desired printing speed, the energy beams canprovide powers of up to 10 mW, 100 mW and even 250 mW or higher. Onecannot therefore merely scan the imaging surface with a single laserbeam, so as to expose the pixels sequentially. Instead, the imagingdevice is required to have a plurality of laser emitting elements forvarious pixels (picture elements) each laser capable of tracing a lineof pixels in the image area of an imaging surface in relative motion.

To achieve acceptable print quality, it is important to have as high apixel density as possible. A high resolution image, for example onehaving 1200 dpi (dots per inch), requires a density of laser emittingelements that is not achievable if the laser emitting elements all liein a straight line, due to the amount of overlap necessary between thelaser sources to achieve a uniform printing quality. Aside from the factthat it is not physically possible to achieve such a high packingdensity, adjacent elements would interfere thermally with one another.

Semiconductor chips are known that emit beams of laser light in an arrayof M rows and N columns. In U.S. Pat. No. 7,002,613 the rows and columnsare exactly perpendicular to each other but the chips are mounted askew,in the manner shown in FIG. 1 of the latter patent, so that each row canfill in the missing pixels of the preceding row(s). In this way, such anarray can achieve a high resolution image but only over the width of thechip and such chips cannot simply be mounted side by side if one is toachieve a printed image without stripes along its length, because thechips cannot have laser emitting elements positioned sufficiently closeto their lateral edges.

U.S. Pat. No. 7,002,613 avoids this problem by arranging such chips intwo rows, in the manner shown in FIG. 8 of the latter patent. The chipsin each row are staggered relative to the chips in the other row of thepair so that each chip in one row scans the gap left unscanned by thetwo adjacent chips in the other row.

Even though it is expected that the rows of chips will be mounted on asupport under clean laboratory conditions using a microscope to achievetheir correct alignment, it is guaranteeing that the relative alignmentof the chips in the two rows will be accurate within the resolution ofthe printed image is difficult and expensive. Any misalignment willresult in the image having stripes or other undesired defects.

US 2010/080594 and US 2008/181667 describe systems in which the lightfrom arrays of LED's (rather than laser sources) is projected onto animage surface and teach how steps may be taken to compensate for anymisalignment between the arrays. In each case, the images produced byadjacent arrays are overlapped and selected LED's from one or other ofthe two arrays are activated to maintain image continuity at theboundary between the two arrays. In the case of US 2010/080594 thisoverlap is shown clearly in FIG. 14 and in US 2009/181667 it is evident,for example, from FIGS. 9A and 9B.

SUMMARY

In the present disclosure, there is disclosed an imaging device forprojecting individually controllable laser beams onto an imaging surfacethat is movable relative thereto in a reference X-direction, the deviceincluding a plurality of semiconductor chips each of which comprises aplurality of individually controllable laser beam emitting elementsarranged in a two dimensional main array of M rows and N columns (M·N),the elements in each row having a uniform spacing A_(r) and the elementsin each column having a uniform spacing a_(c), wherein the chips aremounted on a support in such a manner that when nominally placed, eachpair of chips that are adjacent one another in a reference Y-direction,transverse to the X-direction, are offset from one another in theX-direction, and such that the center of laser beam emitting elements ofthe main M·N emitting elements arrays of both chips in the pair arenominally uniformly spaced in the Y-direction by a nominal distanceA_(r)/M, without overlap in the Y-direction between the beam emittingelements of the adjacent chips. Stated differently, were all the laseremitting elements of the pair of nominally placed adjacent chips to beactivated continuously, and were the chips and imaging surface to be inrelative motion in the X-direction, the emitted laser beams of therespective main arrays of the two chips of the pair would trace on theimaging surface a set of parallel lines that extend in the X-directionand that are nominally uniformly spaced in the Y-direction. The linestraceable by emitting elements of the first chip would not interlacewith the lines traceable by emitting elements of the second chip.

As a major object of the invention involves compensating for minormisalignment of the chips, it is important to realize that thedisclosure of relative placement relates to the desired positioningwithin certain tolerances that enables satisfactory results from theimaging device. Therefore, the term “nominally”, should be construed tomean that the stated spatial relationship exist when the chips or otherrelevant elements are disposed at their intended placing. However,different aspects of the invention allow compensating for chipplacements that diverge from that nominal position. Similarly, when usedto indicate spatial relationship the term “beam” should be considered asrelating primarily to the center of the beam, unless otherwise indicatedor clear from the context. Thus by way of example the uniform spacingA_(r) and a_(c) relate to the distance between the centers of the laserbeam emitting elements.

In order to compensate for minor misalignment, in addition to the M rowsand N columns of elements of the main array, each chip comprises atleast one additional column on at least one side of the main array, eachsuch additional column containing at least one selectively operablelaser emitting element disposed for tracing at least one additional linethat lies between the two sets of M·N lines. This element, also termedthe additional element or the alignment element, is thus capable ofcompensating for some misalignment in the Y-direction in the relativepositioning of the adjacent chips on the support.

Assuming that the M rows and N columns of laser emitting elements of themain array do not include any elements that are normally redundant, thespacing between adjacent lines in the set will be equal to A_(r)/M,namely the spacing of the adjacent elements in each row divided by thenumber of rows. Furthermore, because in the present disclosure there isno overlap between the two sets of M·N lines traced by any two adjacentchips, the total number of lines traced by the two chips will be equalto 2·M·N, namely twice the product of the number of rows and the numberof columns in each chip, if the chips have equal numbers of rows andcolumns.

In an aspect of the invention, in addition to these evenly spaced linesproduced by the main M·N arrays, additional laser emitting elements areprovided on at least one end, or on both ends of each array, intendedonly to compensate for chip misalignment. If adjacent chips arecorrectly aligned, the elements of the additional columns will beredundant and will not be energized. However, if a gap should remainbetween the lines traced by adjacent chips, the additional elements canintroduce additional lines to fill that gap at a position approximatingthe uniform spacing of the lines traced by the main M·N arrays. Itshould be noted that, in contrast to the proposals in US 2010/080594 andUS 2008/181667, the lines traced by the additional elements do not fallbetween (i.e. are not interlaced with) the lines of either set of M·Nlines traced by the main arrays and only fall within any gap between thetwo sets of M·N lines.

Were the imaging surface moved relative to laser beams emitted byadjacent laser elements, the laser radiation centered on each linetraced in the X-direction, would have a non-uniform energy profile whichtypically, but not necessarily, approaches a Gaussian intensitydistribution. The spot size traced can be made large enough so that theenergy traced by one laser element overlaps the area traced by anadjacent element and the intensity combination of the two beams, as wellas the control over the amplitude of one or both beams, offers acombined intensity profile whose maximum may be moved between the twoadjacent traced lines by controlling the relative intensity, and/ortiming, thus placing an intermediate line traced at a selectableposition between the two original line centers.

In the event of an overlap between the two sets of M·N lines traced bythe elements of adjacent chips, some of the elements of the main arrayscan be switched off and if necessary replaced by an element of theadditional columns to maintain the appearance of a raster with uniformlyspaced lines.

On the other hand, if a gap exists between the two M·N lines traced bythe elements of adjacent chips, the additional columns can be activatedto maintain the appearance of a raster with uniformly spaced lines.

One of the additional elements may be activated on its own if itsposition coincides with a line that would render the raster uniform.Alternatively, if the elements have a symmetrical energy profile,resembling for example a Gaussian or a sinusoidal distribution, it ispossible, by activating two elements to irradiate adjacent spots on theimaging surface and separately adjusting the power of each element, toproduce a single raster line at an adjustable distance from the rasterlines of the main elements of the two chips. It should be noted thatthis effect is thermally dynamic and additive provided that the adjacentspots are irradiated within a finite time of each other. In other words,the imaging surface should not have time to dissipate the energy of thefirst laser pulse in the interval between the two laser pulses.Furthermore, the two elements may be either on a single additionalcolumn of one chip or on additional columns each residing on a separatechip, assuming that the additional columns are disposed between therespective main arrays of the two chips. Chips having additional columnson both sides of the main arrays would provide such arrangement of theadditional columns of the two chips being disposed between therespective chips main arrays.

Conveniently, the lines traced by elements in the additional column areevenly spaced from one another, the spacing between the lines traced bythe element of the additional column being substantially equal to thequotient of the spacing of the lines traced by the elements of mainarray and the number of elements in the additional column.

While it would be possible to use chips in which the rows and columns ofthe main arrays of laser emitting elements are perpendicular to oneanother, as taught in U.S. Pat. No. 7,002,613, doing so requires thechips to be placed at an angle relative to the Y-direction. In someembodiments of the present disclosure the elements in each row of eachchip lie on a line parallel to the Y-direction and the elements in eachcolumn of each chip lie on a straight line inclined at an angle to theX-direction. In other words, instead of the outline of the array beingsquare, the array is shaped as a parallelogram. This arrangement, whichmay be considered slightly wasteful as far as chip area is concerned,can be advantageous in terms of assembling procedure.

It is convenient for the chips to be arranged in at least one pair ofrows on the support, with corresponding laser emitting elements of allthe chips in each of the two rows lying in line with one another in theY-direction. By “corresponding elements” it is meant that the individuallaser emitting elements of the M·N main array should occupy the same rowand column positions within their respective chips. It is advantageousfor corresponding elements in any group of three chips in the pair ofrows that are adjacent one another in the X and Y-directions to lie atthe apices of congruent equilateral triangles. This arrangementsimplifies the construction of the lens system to focus the laser beamsonto the imaging surface.

It has been found particularly advantageous for all the laser beamsemitted by one chip to be focused on the imaging surface by a commonsingle lens, or a common set of lenses arranged in series, having amagnification M_(o) whose absolute value is greater than or equal to one(1), however magnification lower than one (1) is also explicitlyconsidered. It was found to be even more advantageous if themagnification M_(o) was substantially equal to +1, as that would ensurethat the laser elements can be spaced adequately on the chip even forhigh resolution systems. Stated differently, the image of the array oflaser elements on the imaging surface (i.e. an array of dots) would havethe same size as the array on the chip, though it may be inverted with amagnification of −1. Notably, even if a slight misalignment of thelenses exists, such as GRIN rod (Gradient-Index) lenses, in the XY planeperpendicular to the optical axis of the lens, the position of theilluminated laser spot on the imaging surface will remain unchanged, asit only depends on the position of the laser emitting element on thelaser array chip. The former elements can be positioned with very highaccuracy on every laser array chip using standard semiconductormanufacturing techniques.

While the lens system may comprise a single GRIN rod associated witheach chip, it may alternatively comprise a plurality of GRIN rodsarranged in series with one another and forming a folded light pathwhere the fold is in the space where a beam emitted by the laserelements is substantially individually collimated. In folded light pathembodiments, a reflecting member such as a prism or mirror which isoptionally common to all the chips may serve to direct the laser beamsfrom one GRIN rod element to the next in each series. In such a foldedlight path configuration, it is desirable for the reflecting member tobe on a facet of a folding prism made of a material, typically a glass,having a higher refractive index than the highest refractive index inthe GRIN rods. The higher index of refraction of the prism will limitthe angular divergence of the collimated beams and allow largerseparation between the sequential GRIN rod segments. A suitable lightpath folding prism can be for example a right angle prism, the foldingface of the prism being a reflecting surface. Other types of reflectingmembers and folding angles can be used depending on the geometry of thesystem and the direction to be given to beams in the series.

It is convenient for the main array of each chip to have an equal numberof rows and columns of laser beam emitting elements (i.e., M=N), as thisminimizes the size of the lens system.

Within each chip, the separation between the laser elements is desirablysufficiently great to minimize thermal interference between adjacentlaser emitting elements.

The support for the chip arrays may be fluid cooled to help dissipatethe heat that may be generated by the chips.

In certain embodiments, the support may be a rigid metallic or ceramicstructure and it may be formed of, or coated with, an electricallyinsulating surface bearing film conductors to supply electrical signalsand power to the chips.

The chips in some embodiments are vertical cavity surface emitting laser(VCSEL) chip arrays. Equivalently other types of laser sources may beutilized and the term VCSEL should be construed as encompassing suchlaser sources.

In some embodiments, the intensity of the laser beam emitted by eachelement may be adjustable either continuously (in an analogue manner) orin discrete steps (digitally). In one embodiment, the chips may includeD/A converters so as to receive digital control signals. In this way,the laser beam intensity may be controllably adjusted in a plurality ofdiscrete steps, such as 2, 4, 8, 16, 32, . . . 4096 and the like.

In a further aspect of the present disclosure, there is provided amethod of projecting individually controllable laser beams onto animaging surface that is movable relative to an imaging device utilizingthe imaging device of any embodiment of the present disclosure, so as toform an image comprising pixels or lines thereof when the projectedlaser beam is intermittent or continuous, respectively.

In some embodiments, at least one pair of laser elements, selectedeither both from the same array or one from each of two adjacent arrays,are controlled in such a manner that their energies are combined on theimaging surface to increase the temperature of the imaging surface abovea predetermined threshold at a point intermediate the centers of theimages of the two laser elements on the imaging surface, without raisingthe temperature of the imaging surface at least one of the centers ofthe images of the two laser elements above the latter threshold.

Clearly in operation the laser emitting elements are switched on and offas needed to provide the required image on the imaging surface, ascontinuous operation of all laser beams would result in a substantiallyuniformly irradiated surface.

BRIEF DESCRIPTION OF THE DRAWINGS

Some embodiments of the imaging device are described herein withreference to the accompanying drawings. The description, together withthe figures, makes apparent to a person having ordinary skill in the arthow the teachings of the disclosure may be practiced, by way ofnon-limiting examples. The figures are for the purpose of illustrativediscussion and no attempt is made to show structural details of anembodiment in more detail than is necessary for a fundamental andenabling understanding of the disclosure. For the sake of clarity andsimplicity, some objects depicted in the figures are not to scale.

In the Figures:

FIG. 1 is a schematic diagram of a digital printing system utilizing animaging device according to an embodiment of the present disclosure;

FIG. 2 shows part of an imaging device comprising a set of VCSEL chipsmounted on a support;

FIG. 3 is a schematic representation of the laser emitting elements oftwo VCSEL chips and the lines that they can trace on a relatively movingimaging surface;

FIG. 4 is a schematic representation that demonstrates in one pair ofrows the alignment between the VCSEL chips and the GRIN rods used aslenses to focus the emitted laser beams onto the imaging surface;

FIG. 5A shows prior art proposals for correction of chip misalignment;

FIG. 5B shows the manner in which an embodiment of the inventioncompensates for chip misalignment;

FIG. 6 shows the energy profiles produced by the laser elements at theends of two adjacent arrays, to illustrate how a single line can betraced using two laterally positioned laser elements, there being shownfor each array three elements of the main array and one of theadditional elements;

FIG. 7A is a similar energy diagram to FIG. 6 to show how the energiesof two adjacent laser elements of the main array can be combined on theimaging surface to produce an additional dot that does not fall on thecenter line of either of the laser elements;

FIG. 7B shows the dot pattern on the imaging surface produced byactivating four laser elements of the main array in the manner shown inFIG. 7A;

FIG. 8A shows how the dot pattern of FIG. 7B assists in anti-aliasing;

FIG. 8B shows for comparison with FIG. 8A the jagged edge that normallyoccurs when printing an oblique line; and

FIG. 9 shows an alternative lens system to that shown in FIG. 1 that hasa folded light path to permit more compact packaging in a printingsystem.

DETAILED DESCRIPTION

The imaging device will be described herein mainly by reference to itsapplication in digital printing systems however its use is not limitedto this application, and different aspects of the invention may beimplemented to controllably project image forming light beams onto anysurface with relative motion between the surface and the chips.

Overall Description of an Exemplary Printing System

FIG. 1 shows a drum 10 having an outer surface 12 that serves as animaging surface. As the drum rotates clockwise, as represented by anarrow, it passes beneath a coating station 14 where it acquires amonolayer coating of fine particles. After exiting the coating station14, the imaging surface 12 passes beneath an imaging device 15 of thepresent disclosure where selected regions of the imaging surface 12 areexposed to laser radiation which renders the particle coating on theselected regions of the surface 12 tacky. Next, the imaging surfacepasses through an impression station 19 where a substrate 20 iscompressed between the drum 10 and an impression cylinder 22. Thepressure applied at the impression station causes the selected regionsof the coating on the imaging surface 12 that have been rendered tackyby exposure to laser radiation by the imaging device 15 in thecorrespondingly termed imaging station to transfer from the imagingsurface 12 to the substrate 20.

The term “tacky” as used herein is intended to mean that the irradiatedparticle coating is not necessarily tacky to the touch but only that itis softened sufficiently to be able to adhere to the surface of asubstrate when pressed against it in the impression station 19.

The regions on the imaging surface 12 corresponding to the selectedtacky areas transferred to the substrate 20 consequently become exposed,being depleted by the transfer of particles. The imaging surface 12 canthen complete its cycle by returning to the coating station 14 where afresh monolayer particle coating is applied only to the exposed regionsfrom which the previously applied particles were transferred to thesubstrate 20 in the impression station 19.

Advantageously, a monolayer of particles facilitates the targeteddelivery of radiation as emitted by the laser elements of an imagingdevice according to present teachings. This may ease the control of theimaging device and process, as the selectively irradiated particlesreside on a single defined layer. When considered for use in a printingsystem, an imaging device targeting a monolayer can preferably focus thelaser radiation to form upon transfer to a substrate a dot ofapproximately even thickness and/or relatively defined contour.

Reverting to the coating station 14, it may comprise a plurality ofspray heads 1401 that are aligned with each other along the axis of thedrum 10 and only one is therefore seen in the section of FIG. 1. Thesprays 1402 of the spray heads are confined within a bell housing 1403,of which the lower rim 1404 is shaped to conform closely to the imagingsurface leaving only a narrow gap between the bell housing 1403 and thedrum 10. The spray heads 1401 are connected to a common supply rail 1405which supplies to the spray heads 1401 a pressurized fluid carrier(gaseous or liquid) having suspended within it the fine particles to beused in coating the imaging surface 12.

The imaging device 15 in FIG. 1 is composed of a support 16 carrying anarray of chips each having an arrangement of individually controlledlaser sources capable of emitting laser beams. In some embodiments, thelaser beam emitting elements can coherently emit light in a range ofwavelengths from about 400 nm to about 12 μm, or up to about 10 μm, orup to about 8 μm, or up to about 3 μm, or up to about 1.4 μm. Suchranges includes regions generally known as Near Infra Red (NIR,˜0.75-1.4 μm), Short-Wavelength Infra Red (SWIR, ˜1.4-3 μm),Mid-Wavelength Infra Red (MWIR), also called Intermediate Infra Red(IIR, 3-8 μm), and Long-Wavelength Infra Red (LWIR, 8-15 μm), also knownas Thermal Infra Red (TIR). In a particular embodiment, the laser beamemitting elements are NIR lasers. The laser sources may by way ofexample, be of VCSEL (Vertical Cavity Surface Emitting Laser) type,however other types may be utilized. By way of example, semiconductorlasers commercially available as laser diodes are capable of emitting atwavelengths from 375 nm to 3,500 nm, covering most of NIR and SWIRregions of the spectrum. Gas lasers can emit over various area of thespectrum, depending on the elected gas and some optical design.Commercial carbon dioxide (CO₂) lasers, for instance, can emit hundredsof watts in the thermal infrared region at 10.6 μm. While for brevitythe term VCSEL is predominantly used herein, it should be construed asencompassing any such laser sources which may be better suited forcertain embodiments.

Each chip has individually controllable laser beam emitting elementsarranged in a two dimensional main array of M rows and N columns (M·N),the elements in each row having a uniform spacing A_(r) and the elementsin each column having a uniform spacing a_(c). As disclosed below, atleast one additional column is also provided.

Preferably, the chips can be individually or collectively associatedwith an array of corresponding lenses 18 that focus the laser beams onthe imaging surface 12 is also used. FIGS. 2 to 4 provide more detailsof the chips 30 according to some embodiments of the invention and onthe manner in which they can be mounted on the support and aligned withthe lenses 18.

FIG. 2 shows a support 16 on which are mounted a plurality of VCSELchips 30 arranged in two rows in accurately predetermined positionsrelative to one another, as will be described in more detail byreference to FIGS. 3 and 4.

The support 16 is a rigid and in some embodiments at least partiallyhollow elongate body fitted with connectors 34 to allow a cooling fluidto flow through its internal cavity. In some embodiments, the body ofthe support may be made of an electrically insulating material, such asa suitable ceramic, or it may be made of a metal and at least itssurface 36 on which the chips 30 are mounted may be coated with anelectrical insulator. This enables a circuit board made of thin filmconductors (partial and symbolic depiction of the conductors isschematically shown to the lower-right chip at FIG. 2) to be formed onthe surface 36. The chips 30 are soldered to contact pads on thiscircuit board and a connector 32 projecting from the lower edge of thesupport 16 allows control and power signals to be applied to the chips30. The laser emitting elements 40 of each chip 30 are individuallyaddressable and are spaced apart sufficiently widely to minimize thermalinterference with one another.

In some embodiments, the individually controllable laser elements of achip can emit laser beams having variable energy that is preferablydigitally controllable in discrete steps, allowing the laser intensityto be set at discrete levels such as 2, 4, 8, 16 . . . and the like, andin some embodiments individual laser beam sources may be controllablyset to emit up to 4096 levels or more. The lowermost level of energy isdefined as 0, where the individual laser element is not activated, theuppermost level of energy can be defined as 1. The distinct intermediatelevels therebetween may be considered analogous in the field of printingto “grey levels”, each level providing for a gradually distinctintensity (e.g., shade when considering a colored output). Taking forinstance, a laser beam emitting element having 16 levels of activation,level 0 would result in lack of impression (e.g., leaving a substratebare or white if originally so) and level 1 would result in transfer ofa tacky film formed by a particle irradiated at maximum energy (e.g.,forming a full black dot in the event the particles are so colored). Inprevious illustrative example, levels 1/16, 2/16, 3/16 and so on wouldcorrespond to increasingly stronger shades of grey, comprised betweenwhite (0) and black (1). Typically, the energy levels are evenly spaced.

In an alternative embodiment, the individually controllable laserelements of a chip can emit laser beams having variable energy that canbe modulated in a continuous analogue manner.

Once a region of the imaging surface has reached a temperature at whichthe particles become tacky, any further increase in temperature will nothave any effect on the transfer to the substrate. However, it shouldalso be noted that as the intensity of the laser is increased the sizeof the dot that is rendered tacky also increases.

The energy profile of each dot resembles the plots shown in FIG. 6, thatis to say that it is symmetrical with tapering sides. The exact profileis not important as the distribution may be Gaussian, sinusoidal or evenan inverted V. In any such profile, as the peak intensity increases, thebase widens and the area of intersection of the profile with a thresholdat which the particle coating is rendered tacky also increases indiameter. A consequence of this energy distribution is that points ofthe imaging surface that are not in alignment with the centerline of anyone laser emitting element will receive energy from adjacent elements.It is possible for two nearby elements to be energized to below thelevel needed to render coating particles on the centerline of theelements tacky, yet for the cumulative energy in the region of overlapbetween the two centerlines to rise above the level necessary to renderthe coating particles tacky. In this way, it is possible to createpotential raster lines between the centerlines of the laser lines inaddition to, or as an alternative to, the raster lines coinciding withthe centerlines of the laser elements. This ability to combine theenergies from adjacent elements is used to achieve different effects, aswill be described below. These effects are dependent upon the ability ofthe imaging surface to combine energies received from different laserelements, even if there is a slight difference between the times ofirradiation.

FIG. 3 shows schematically, and to a much enlarged scale, the relativepositioning of two laser emitting element arrays 130 a and 130 b ofchips 30 that are adjacent one another in the Y-direction but arelocated in different rows. Each of the chips has a main array of M by Nlaser emitting elements 40, as previously described, which arerepresented by circular dots. In the example illustrated, M and N areequal, there being nine rows and nine columns. The spacing between theelements in a row, designated A_(r), and the spacing between theelements in a column, designate a_(c), are shown as being different fromone another but they may be the same. The array is shown as beingslightly skewed so that the columns and rows are not perpendicular toone another. Instead, the rows lie parallel to the Y-direction while thecolumns are at a slight angle to the X-direction. This enables lines,such as the lines 44, traced by the elements 40 on the imaging surface,if energized continuously, to be sufficiently close together to allowhigh resolution images to be printed. FIG. 3 shows that the element atthe end of each row traces a line that is a distance A_(r)/M away fromthe line traced by the corresponding element of each adjacent row, theseparation between these lines being the image resolution I_(r). Thus,assuming a magnification of |1|, A_(r) and M are selected in dependenceupon the desired image resolution, based on the equation A_(r)=M·I_(r).

It should be mentioned that it is possible for the elements to lie in asquare array where the columns are perpendicular to the rows. In thiscase, the chips would need to be mounted askew on their support andcompensation would need to be applied to the timing of the controlsignals used to energize the individual elements.

As is clear from FIG. 3, and also FIG. 5B which shows the traced linesto a larger scale, the positioning of the array 130 b is such that theline traced by its bottom left element 40 should ideally also be spacedfrom the line traced by the top right element of the array 130 a by adistance equal to A_(r)/M. Therefore when all the elements 40 of botharrays 130 a and 130 b are energized, they will trace 2·M·N lines thatwill all be evenly spaced apart by a distance A_(r)/M between adjacentlines, without any gaps.

If one wishes to provide compensation for defective elements, the arraycould include additional rows of laser emitting elements 40, but it isalternatively possible to compensate for a defective element byincreasing the intensity of the laser beams generated by the laseremitting elements that trace the two adjacent parallel lines.

In addition to the M by N array of elements 40, each chip has at leastone additional column that is arranged along the Y-direction on the sideof the main array, the additional column containing at least one laserbeam emitting element 42. These further elements 42 are represented inFIG. 3 by stars, to distinguish them from the main array elements 40. Asseen in FIG. 4, in some embodiments at least two such additional columnseach of one element 42 are provided, at least one column disposed in Ydirection on each side of the main N by M array. The additional laserelements of the additional columns on one or both sides of each mainarray can be respectively positioned at a distance of ½ or ⅓ the spacingbetween traced lines that can be imaged by the lenses onto the imagingsurface. Furthermore additional elements could be placed in the gapbetween two arrays that nominally spans a distance of A_(r)/M so thathigher sensitivity is achieved in correcting the spacing errors betweenadjacent arrays.

Any additional element 42 of an additional column can be positioned inthe column at any desired distance from the edge element of the mainarray, the distance in the Y-direction depending on the total numbers ofadditional elements/additional columns each two sets of main arrays of apair of chips to be aligned would bound. Assuming n additional elements42 between a first and second main array, n being a positive integernumber, each additional element can be spaced from the edge element ofthe main arrays or from one another in the Y-direction by a distanceequal to A_(r)/(n+1), namely the spacing of the adjacent elements ineach row divided by one more than the number of additional elements inthe gap. Considering now the X-direction, the additional elements caneither be aligned with a row of elements of their respective main arraysor positioned at any desired intermediate position above or below suchrows. Preferably the positioning of an additional element 42 withrespect to adjacent elements of the main array shall minimize thermalinterference. Notably, the additional element or elements may bedisposed at any position along the X-direction of the chip.

In practice n elements 42 positioned in any of the additional columns onone or both sides of the main array, can correct for alignment errors ofup to about a 1/(n+1) of the nominal spacing between the edge elementsof two adjacent chips. If, by way of example, the edge elements of thetwo chips are at a distance of 20 μm (micrometers) in the Y-direction,and there is a single additional laser emitting element on adjacentsides of each array, such elements may correct a spacing error of up toabout one third of the nominal spacing, in the exemplified caseapproximately 7 μm. Any positional deviation from the desired positionon the chip (e.g., with respect to its edges) or nominal distancebetween elements not exceeding 10%, is considered within tolerances,however in most cases due to the high precision of the semiconductormanufacturing methods, such errors are unlikely.

As can be seen from FIG. 3 and FIG. 5B, when activated, these elements42 trace two additional lines 46 between the two sets of evenly spacesparallel lines 44 a and 44 b traced by the elements 40 of the two arrays130 a and 130 b, respectively.

One of the additional lines 46 is spaced by a distance A_(r)/3M from thelast adjacent line 44 a traced, for example, by the array 130 a in FIG.3 and the other is spaced by a distance A_(r)/3M from the first adjacentline 44 b traced, for example, by the array 130 b. In the event of amisalignment between the two arrays 130 a and 130 b these elements 42can be energized in addition to, or instead of some of, the elements 40of the main arrays to compensate for any misalignment between the arrays130 a and 130 b that tends to create a stripe in the printed image, beit a gap or a dark line resulting from an unintentional overlap. FIG.5A, which is similar to FIG. 5B, shows the alternative approach proposedin the prior art to compensate for chip misalignment. In the prior art,each chip has an additional row of elements that produces traced linesthat are interlaced with the traced lines of the adjacent chip,resulting in a very high degree of redundancy.

While the two additional elements 42 in the present embodiment are shownin FIG. 3 and FIG. 5B as tracing two separate lines 46, the energies ofthese two elements can be combined on the imaging surface, as earlierdescribed, to form a single line of which the position is controllableby appropriate setting of the energies emitted by each of the additionalelements 42. This is shown in FIG. 6 in which the energy profiles of thelines 44 a and 44 b are designated 94 a and 94 b, respectively and theenergy profiles of the additional lines 46 are designated 96 a and 96 b.In FIG. 6, neither of the profiles 96 a and 96 b (shown in dotted lines)has sufficient energy to render the coating particles tacky but at thecenterline between the two arrays the cumulative energy, shown as asolid dark line 96, is sufficient to soften the particles coating and tocreate a trace line filling the gap between the trace lines 44 a and 44b of the two main arrays.

While in FIG. 6 the energy profiles of the two additional elements arematched, it is possible by varying the relative intensity of the twobeams emitted by the additional laser sources to position the centerlineof the combined energy at a different distance from the traces of themain arrays.

FIG. 7A shows how the ability to create dots that do not fall on thecenterlines of the energy profiles of the laser elements can be used toadvantage to achieve anti-aliasing. FIG. 7A shows the energy profiles offour adjacent elements of the main array. The first two profiles a and bare set at a desired level, say 8 (out of sixteen), corresponding tomid-grey. The energy profiles c and d, on the other hand are set to say12 and 4, respectively. The resulting dot pattern produced on theimaging surface is shown in FIG. 7B. This can be seen to comprise tworegular sized dots A and B aligned with the line of symmetry of theprofiles a and b in FIG. 7A, a larger sized dot C aligned with thecenterline of energy profile c, and a smaller dot D that lies somewherebetween the centerlines of the profiles c and d.

The result of repeating such a dot pattern diagonally is shown in FIG.8A. When this image is compared with FIG. 8B, where no anti-aliasingsteps have been taken, it will be seen that the small dots in betweenregular raster line yield oblique edges that have reduced jaggedness andproduce an image that is comparable with one achievable by a printingsystem having a greater image resolution.

The interaction of energies from nearby laser elements can also be usedto compensate for missing or inoperative elements in that the elementsproducing the two adjacent raster lines can be used to combined in thesame manner as previously explained to fill in a gap between them.

For the arrays 130 a and 130 b in FIG. 3 to function correctly asdescribed above, their relative position in the Y-direction is veryimportant. In order to simplify the construction of the lens systemserving to focus the emitted laser beams on the imaging surface it isadvantageous to adopt a configuration shown in FIG. 4 which enables thetwo rows of lenses corresponding to a pair of chip rows to beself-aligning.

FIG. 4 shows arrays of seven adjacent chips 130 each shown lined up witha respective lens 18. Additional laser elements 42, on each side of themain array of each chip, are also schematically illustrated in thefigure. Each lens 18 is constructed as a GRIN (Gradient-Index) rod, thisbeing a known type of lens that is shaped as a cylinder having aradially graduated refractive index. In the case of the geometry shownin FIG. 4, the respective centers of corresponding elements of any threebi-directionally adjacent chip arrays 130 lie nominally on the apices ofan equilateral triangle, three such triangles designated 50 being shownin the drawing. It will be noted that all the triangles 50 arecongruent. As a result, if the diameter of the GRIN rods is now selectedto be equal to 2·N·A_(r), which is the length of the sides of theequilateral triangles 50, or the distance between corresponding laseremitting elements of adjacent VCSEL chips 30 in the same row, then whenstacked in their most compact configurations, after aligning the lensarray to the Y-direction over the chips, the lenses 18 willautomatically align correctly with their respective chip. For suchconstruction, the relationship between the rod lens diameter D, theimage resolution I, and the size of the matrix of laser elements is:D=2I_(r)·M·N where I, is the spacing in the Y-direction between adjacentlines traceable in the X-direction and M is the number of rows and N thenumber of columns in the main M·N array, assuming absolute magnificationvalue of |1|.

Though the lens 18 has been schematically illustrated in FIG. 1 (sideview) and FIG. 4 (cross section view) as being an individual GRIN rod,in alternative embodiments the laser beams of each chip can betransmitted by a series of lenses. In the simplified embodiment shown inFIG. 9, the single GRIN rod 18 is replaced by two mutually inclined GRINrods 18 a and 18 b and the light from one is directed to the other by areflecting member which in the example of FIG. 9 is embodied by a prism87 of high refractive index glass, so that the light follows a foldedpath. It is noted that other reflecting members such as mirrors and thelike may be utilized. Such a configuration enables coating stations in acolour printing system to be arranged closer to one another in a morecompact configuration. Such a folded light path can adopt differentconfigurations while fulfilling all the requirements of magnificationand light transmission. To enable the light path to be split in thismanner, the length of the GRIN rods is preferably selected such thatlight beams are individually collimated on leaving the rods 18 a andentering the rods 18 b as shown by the light rays drawn in FIG. 9.

The radiation guided by GRIN rod 18 a, the proximal end of which isarranged at a distance WD_(o) from the chip, may be captured by thecorresponding GRIN rod 18 b which can collect the collimated lightemerging from rod 18 a on the same light path and focus it at a distanceWD_(i) from the distal end of the second GRIN rod 18 b. When the twoGRIN rods are made of the same material and the same radial gradientprofile and WD_(o)=WD_(i) a magnification of M_(o)=+1 or −1 can beobtained.

Notably, with straight or folded path light paths, the magnificationshould be considered substantially equal to its nominal value if within±0.5% or even 1% or 2%.

Laser elements that are away from the longitudinal axis of the GRIN rod18 a will leave the distal end of the GRIN lens collimated but at anangle to the axis. In certain cases, it is necessary for the distancebetween the two rods 18 a and 18 b to be large, causing the off axiscollimated beams exiting the first rod segment to miss partially orentirely the second segment. It is possible to take advantage of Snell'slaw and cause the beam exiting the first rod to travel through a glasswith a high refractive index, thus causing the angle the collimated beammakes with the optical axis to decrease and enabling a larger separationbetween the rods before the collimated beams leaving the first rod missthe entrance to the second rod.

In the description and claims of the present disclosure, each of theverbs, “comprise” “include” and “have”, and conjugates thereof, are usedto indicate that the object or objects of the verb are not necessarily acomplete listing of members, components, elements, steps or parts of thesubject or subjects of the verb.

As used herein, the singular form “a”, “an” and “the” include pluralreferences and mean “at least one” or “one or more” unless the contextclearly dictates otherwise.

Positional or motional terms such as “upper”, “lower”, “right”, “left”,“bottom”, “below”, “lowered”, “low”, “top”, “above”, “elevated”, “high”,“vertical”, “horizontal”, “backward”, “forward”, “upstream” and“downstream”, as well as grammatical variations thereof, may be usedherein for exemplary purposes only, to illustrate the relativepositioning, placement or displacement of certain components, toindicate a first and a second component in present illustrations or todo both. Such terms do not necessarily indicate that, for example, a“bottom” component is below a “top” component, as such directions,components or both may be flipped, rotated, moved in space, placed in adiagonal orientation or position, placed horizontally or vertically, orsimilarly modified.

Unless otherwise stated, the use of the expression “and/or” between thelast two members of a list of options for selection indicates that aselection of one or more of the listed options is appropriate and may bemade.

The imaging device is described herein mainly by reference to itsapplication in digital printing systems however its use is not limitedto this application, and different aspects of the invention may beimplemented to project light beams onto any surface with relative motionbetween the surface and the chips.

In the disclosure, unless otherwise stated, adjectives such as“substantially” and “about” that modify a condition or relationshipcharacteristic of a feature or features of an embodiment of the presenttechnology, are to be understood to mean that the condition orcharacteristic is defined to within tolerances that are acceptable foroperation of the embodiment for an application for which it is intended.For instance, each two adjacent elements of the group of elements underconsideration (such as by way of example of a chip row, of a chipcolumn, or of adjacent chip arrays, when applicable) are considered“substantially uniformly spaced” if the deviation of each pair ofadjacent elements from a desired nominal distance does not exceed 10% ofthis predetermined spacing. Pairs of adjacent elements deviating fromthe nominal distance by less than 5%, 4%, 3%, 2% or 1% are furtherconsidered “substantially uniformly spaced” or “having a substantiallyuniform spacing”. By way of example, assuming a desired A_(r)=20micrometers, and the desired nominal spacing in the Y-direction betweencorresponding main array laser emitting elements in two adjacent chipsequals A_(r)·N, spacing deviations resulting from manufacturingtolerance of no more than 2 μm, are considered to fall within thenominal spacing. Clearly, smaller or no deviations are desired.

While this disclosure has been described in terms of certain embodimentsand generally associated methods, alterations and permutations of theembodiments and methods will be apparent to those skilled in the art.The present disclosure is to be understood as not limited by thespecific embodiments described herein.

The invention claimed is:
 1. An imaging device for projectingindividually controllable laser beams onto an imaging surface, theimaging device and imaging surface being movable relative to each otherin a reference X-direction, the imaging device comprising: a pluralityof semiconductor chips each of which comprises a plurality ofindividually controllable laser beam emitting elements arranged in a twodimensional main array of M rows and N columns, the emitting elements ineach row having a uniform spacing Ar and the emitting elements in eachcolumn having a uniform spacing ac; wherein the chips are mounted on asupport such that the main arrays of each pair of chips that areadjacent one another in a reference Y-direction, transverse to theX-direction, are offset from one another in the X-direction, whereinwhen the chips are nominally placed, were all the emitting elements ofthe main arrays of the two chips to be activated continuously and theimaging surface and the imaging device relatively moved in theX-direction, the emitted laser beams of the respective main arrays ofthe two chips of the pair, would trace on the imaging surface 2·M·Nparallel lines that extend in the X-direction and are uniformly spacedfrom one another in the Y-direction by a nominal distance Ar/M, wherebythe laser beams of each chip tracing a set of M·N lines withoutoverlapping the set of lines of the other chip; and, each chip furthercomprises at least one additional column in addition to the N columns ofelements of the main array, the additional column being disposed on atleast one side of the main array along the Y-direction, and containingat least one selectively operable laser emitting element capable oftracing at least one additional line that lies between the respectivesets of M·N lines of each pair of chips and that is passed from twoadjacent lines, each from a respective one of the sets, by a distancesmaller than the uniform element spacing in each row Ar.
 2. An imagingdevice as claimed in claim 1, wherein the at least one additional columncomprises a plurality of elements.
 3. An imaging device as claimed inclaim 2, wherein when the imaging surface and the imaging device arerelatively moved in the X-direction and the elements of the additionalcolumn are continuously activated, the resulting laser beams tracenominally uniformly spaced lines, the spacing between the lines tracedby the elements of additional columns and disposed between lines tracedby elements of the main arrays being substantially equal to the quotientof the spacing of the lines traced by the elements of the main array(Ar/M) and one more than the total number of elements in the additionalcolumns.
 4. An imaging device as claimed in claim 1, wherein theelements in each row of each chip lie on a line parallel to theY-direction and the elements in each column of each chip lie on astraight line inclined at an angle to the X-direction.
 5. An imagingdevice as claimed in claim 1, wherein the chips are arranged in pair ofrows on the support and corresponding laser emitting elements of all thechips in each of the two rows lying in line with one another in theY-direction.
 6. An imaging device as claimed in claim 5, furthercomprising a plurality of lens systems, each serving to focus the laserbeams of all the laser elements of a respective one of the chips ontothe imaging surface, each lens system comprising at least one gradientindex (GRIN) rod; and, the chips within the two rows of the pair beingaligned such that corresponding elements in any group of three adjacentchips in the X and Y-directions lie at the apices of congruentequilateral triangles and the distance in the Y-direction betweencorresponding elements is nominally equal to N·Ar.
 7. An imaging deviceas claimed in claim 6, wherein the GRIN rod is of circular cross-sectionhaving a diameter equal to 2·N·Ar.
 8. An imaging device as claimed inclaim 6, wherein the lens systems have a magnification of absolute valueequal or greater than
 1. 9. An imaging device as claimed in claim 6,wherein the lens systems magnification value is +1 or −1.
 10. An imagingsystem as claimed in claim 6, wherein each lens system is formed by aplurality of mutually inclined GRIN rods.
 11. An imaging system asclaimed in claim 10, wherein light from each GRIN rod is directed to thenext GRIN rod in the plurality of rods of the same lens system by atleast one reflecting member.
 12. An imaging system as claimed in claim11, in which the reflecting member is a prism having a higher refractiveindex than the highest refractive index of the GRIN rods.
 13. An imagingdevice as claimed in claim 1, wherein each chip comprises at least asecond additional column, such that at least one additional column isdisposed on each side of the respective main array.
 14. An imagingdevice as claimed in claim 1, wherein the support is fluid-cooled. 15.An imaging device as claimed in claim 1, wherein the support isconstructed of a rigid metallic or ceramic structure.
 16. An imagingdevice as claimed in claim 1, further comprising at least one lensassociated with each chip of the pair of chips, wherein the respectivelens focuses the laser beams emitted by all the elements of theassociated chip onto the imaging surface.
 17. An imaging device asclaimed in claim 1, wherein each chip has an equal number of rows andcolumns of laser beam emitting elements in the main array.
 18. Animaging device as claimed in claim 1, wherein the surface of the supportis formed of, or coated with, an electrical insulator, and furthercomprising a plurality of thin film conductors formed on theelectrically insulating surface for supplying electrical signals andpower to the chips.
 19. An imaging device as claimed in claim 1, whereinthe laser emitting elements are a vertical cavity surface emittinglasers (VCSEL) array.
 20. An imaging device as claimed in claim 1,wherein each individually controllable laser beam element can emit alaser beam of at least four selectable levels of energy.